Phytophthora cinnamomi (Phytophthora dieback).

P. cinnamomi is a soilborne pathogen that is now widely established in many parts of the world. Initial long-range spread is likely to have been on infected nursery plants (e.g., Kenerley and Bruck, 1983; Benson and Campbell, 1985; Davison et al., 2006), and still occurs in this way. Additional long-range spread is by movement of soil and gravel infested with chlamydospores (e.g., Batini, 1977; Colquhoun and Petersen, 1994). Short-range spread is also by zoospores in drainage, seepage and irrigation water (Kinal et al., 1993; MacDonald et al., 1994). It has a very wide host range (Zentmyer, 1980) so that, once introduced into an area, it can persist on the roots of many different plants without necessarily causing symptoms on the foliage. It is a major pathogen of horticultural crops, in forestry and in natural vegetation, especially in southern Australia (Natural Resource Management Western Australia, 2013 - see http://www.dieback.net.au/pages/1382/susceptible-species). It is regarded as a key threatening process in the Australian environment (Environment Protection and Biodiversity Conservation Act, 1999), affecting both plant communities by reducing diversity, and the animal communities that depend on them. It is considered by ISSG (2012) to be one of the 100 worst invasive species worldwide.

Augmenting the Sustainability of Vegetable Cropping Systems by Configuring Rootstock-Dependent Rhizomicrobiomes that Support Plant Protection

Herbaceous grafting is a propagation method largely used in solanaceous and cucurbit crops for enhancing their agronomic performances especially under (a)biotic stress conditions. Besides these grafting-mediated benefits, recent advances about microbial networking in the soil/root interface, indicated further grafting potentialities to act as soil environment conditioner by modulating microbial communities in the rhizosphere. By selecting a suitable rootstock, grafting can modify the way of interacting root system with the soil environment regulating the plant ecological functions able to moderate soilborne pathogen populations and to decrease the risk of diseases. Genetic resistance(s) to soilborne pathogen(s), root-mediate recruiting of microbial antagonists and exudation of antifungal molecules in the rhizosphere are some defense mechanisms that grafted plants may upgrade, making the cultivation less prone to the use of synthetic fungicides and therefore more sustainable. In the current review, new perspectives offered by the available literature concerning the potential benefits of grafting, in enhancing soilborne disease resistance through modulation of indigenous suppressive microbial communities are presented and discussed.

Genome Sequences of Verticillium dahliae Defoliating Strain XJ592 and Nondefoliating Strain XJ511

Verticillium dahliae is a widely distributed soilborne pathogen that causes vascular wilt in more than 200 plant species. Defoliating and nondefoliating symptoms caused by the disease that result in either the loss or retention of leaves in infected plants, respectively, in hosts such as cotton, olive, and okra, divide the causal agent into defoliating and nondefoliating pathotypes. Our goal in this current work was to generate genome resources for the defoliating strain XJ592 and the nondefoliating strain XJ511 of V. dahliae isolated from cotton in China.

Genome Sequence of Fusarium oxysporum f. sp. matthiolae, a Brassicaceae Pathogen

The filamentous fungus Fusarium oxysporum is a soilborne pathogen of many cultivated species and an opportunistic pathogen of humans. F. oxysporum f. sp. matthiolae is one of three formae speciales that are pathogenic to crucifers, including Arabidopsis thaliana, a premier model for plant molecular biology and genetics. Here, we report a genome assembly of F. oxysporum f. sp. matthiolae strain PHW726, generated using a combination of PacBio and Illumina sequencing technologies. The genome assembly presented here should facilitate in-depth investigation of F. oxysporum–Arabidopsis interactions and shed light on the genetics of fungal pathogenesis and plant immunity.

Understanding Why Effective Fungicides Against Individual Soilborne Pathogens Are Ineffective with Soilborne Pathogen Complexes

Annual forage legumes across southern Australia continue to be devastated by soilborne diseases. Nine fungicide seed treatments (thiram, metalaxyl, iprodione, phosphonic acid, propamocarb, fluquinconazole, difenoconazole + metalaxyl, ipconazole + metalaxyl, sedaxane + difenoconazole + metalaxyl) and four foliar fungicide treatments (phosphonic acid, metalaxyl, propamocarb, iprodione) were tested on four subterranean clover cultivars against individual oomycete soilborne pathogens Pythium irregulare, Aphanomyces trifolii, and Phytophthora clandestina and the fungal pathogen Rhizoctonia solani. Best treatments were then further tested across southern Australia in 2 years of field experiments. Under controlled conditions, seed treatment with thiram was best against damping-off caused by P. irregulare across the four cultivars (Woogenellup, Riverina, Seaton Park, Meteora), while metalaxyl was the most effective for maximizing root and shoot weights. Against A. trifolii, metalaxyl, iprodione, difenoconazole + metalaxyl, ipconazole + metalaxyl, and sedaxane + difenoconazole + metalaxyl, all reduced damping-off; sedaxane + difenoconazole + metalaxyl, fluquinconazole, and ipconazole + metalaxyl all reduced lateral root disease across two or more cultivars; while iprodione, thiram, and sedaxane + difenoconazole + metalaxyl increased shoot dry weight. Against P. clandestina, metalaxyl was the most effective in reducing tap and lateral root rot followed by ipconazole + metalaxyl or phosphonic acid for tap and lateral rot, respectively. Against R. solani, there were no effects of fungicides. For P. irregulare and P. clandestina, there were strong seed fungicide × cultivar interactions (P < 0.001). Under controlled conditions for foliar fungicide spray treatments, phosphonic acid was best at preventing productivity losses from A. trifolii, but was ineffective against P. clandestina, P. irregulare, or R. solani. Overall, controlled environment studies highlighted strong potential for utilizing seed treatments against individual pathogens to ensure seedling emergence and early survival, with seed and foliar sprays enhancing productivity by reducing seedling damping-off and root disease from individual pathogens. However, in field experiments over 2 years across southern Australia against naturally occurring soilborne pathogen complexes involving these same pathogens, only rarely did fungicide seed treatments or foliar sprays tested show any benefit. It is evident that currently available fungicide seed and/or foliar spray treatment options do not offer effective field mitigation of damping-off and root disease on annual forage legumes that underpin livestock production across southern Australia. The main reason for this failure relates to the unpredictable and ever-changing soilborne pathogen complexes involved, highlighting a need to now refocus away from fungicide options, particularly toward developing and deploying new host tolerances, but also in deploying appropriate cultural control options.

Soil Is Not a Reservoir for Phellinus noxius

Phellinus noxius causes brown root rot (BRR) of diverse trees. Basidiospores and diseased host tissues have been recognized as important sources of P. noxius inoculum. This study aimed to understand whether P. noxius could occur or survive in soil without host tissues in the natural environment. Soil was sampled before and after the removal of diseased trees at eight BRR infection sites (total of 44 samples). No P. noxius colonies were recovered in soil plating assays, suggesting that no or little viable P. noxius resided in the soil. To know whether P. noxius could disseminate from decayed roots to the surrounding soil, rhizosphere and non-rhizosphere soils were sampled from another two infection sites. Although P. noxius DNA was detectable with specific primers, no P. noxius could be isolated, even from the rhizosphere soils around decayed roots covered with P. noxius mycelial mats. The association between viable P. noxius and the presence of its DNA was also investigated using field soil mixed with P. noxius arthrospores. After P. noxius was exterminated by flooding or fumigation treatment, its DNA remained detectable for a few weeks. The potential of onsite soil as an inoculum was tested using the highly susceptible loquat (Eriobotrya japonica). Loquats replanted in an infection site that had been cleaned up by simply removing the diseased stump and visible residual roots remained healthy for a year. Taken together, P. noxius is not a soilborne pathogen, and diseased host tissues should be the focus of field sanitation and detection for BRR.

Transmission of Fusarium oxysporum f. sp. fragariae Through Stolons in Strawberry Plants

Fusarium wilt of strawberry, caused by the soilborne pathogen Fusarium oxysporum f. sp. fragariae, is a growing threat to the strawberry industry worldwide. Symptoms of the disease typically include stunting, wilting, crown discoloration, and eventual plant death. When Fusarium wilt was discovered in California, the disease was not known to occur anywhere else in North America. Long distance movement of the pathogen would most likely occur through transport of infected plants, which seems plausible if strawberry plants can sustain infections without showing symptoms of disease. The results of this study document that F. oxysporum f. sp. fragariae can move through stolons of infected mother plants and colonize first-generation daughter plants. The pathogen can also move through stolons from first to second-generation daughter plants. Daughter plants of both generations were always symptomless. The pathogen was recovered from both roots and petioles of infected daughter plants. Similar results were obtained for two cultivars known to be susceptible to Fusarium wilt, Albion and Monterey. Transmission through stolons from mother to daughter plants also occurred in the resistant cultivar, San Andreas, but less frequently than in Albion and Monterey.